1. Field of the Invention:
The present invention relates to the recovery of vanadium values from compositions/materials comprised thereof, and, more especially, to the recovery of vanadium values contained in used or spent catalysts.
2. Description of the Prior Art:
It is known to this art that vanadium, in particular in the form of vanadium pentoxide V.sub.2 O.sub.5, is presently widely used as a catalyst in numerous chemical processes, for example in the oxidation of sulfur dioxide to sulfur trioxide in the production of sulfuric acid, or in the reduction of the nitrogen oxides contained in residual gases, in the presence of ammonia.
In all of these cases, it is observed, after a more or less extensive period of use, that there is a very marked reduction in the catalytic activity of the catalysts, generally related to chemical and/or mechanical aging phenomena. In the particular case of the production of sulfuric acid, for example, a progressive poisoning of the catalyst is determined due to the partial or total conversion of the vanadium pentoxide.
For this reason, in order to maintain the production rate, it is necessary to replace the used catalysts with fresh material.
In light of the high cost of vanadium on the one hand, the advantage provided by a treatment of residual catalyst material to recover the catalytic elements is readily apparent.
On the other hand, environmental constraints mandate that the residual vanadium values of spent material to be discharged to waste do not exceed 5 g vanadium/kg. Therefore, this presupposes a process providing recovery yields that are compatible with such constraints.
In Czechoslovak Patent No. 178,626, filed June 16, 1975, a process is described for the recovery of vanadium values from residual catalysts based on the leaching, by percolation of the catalytic material with water, in the presence of pure sulfur dioxide. The SO.sub.2 serves as the reducing agent for the vanadium pentoxide contained in the catalyst. On the one hand, a solid filter cake (catalyst support material) is produced, as well as, on the other, a clear vanadyl sulfate filtrate.
However, such a process has the disadvantage of requiring pure sulfur dioxide gas, which is expensive. Thus, such a process is uneconomical for applications on an industrial scale. The '626 patent is conspicuously silent as regards alternate gaseous compounds that may be suitable for use in the process.